Jeremy Lusk, an American freestyle motocross rider, died of head injuries Tuesday after crashing while trying to land a backflip in competition. He was 24.Jorge Ramirez, chief of the intensive care unit at Calderon Hospital, said Lusk suffered severe brain damage and a possible spinal cord injury. Lusk won a gold medal at the 2008 X Games. He was injured Saturday night when he failed to complete a full rotation while attempting a Hart Attack backflip and slammed headfirst into the dirt. Lusk crashed in almost identical fashion in the freestyle semifinals at the 2007 X Games but was not hurt.